Plug-in hybrids may one day constitute a majority of the cars on U.S. roads. Like today’s hybrids, they have both a gasoline engine and an electric-drive motor whose batteries can be recharged by the engine. But they can also be recharged at a standard wall socket. Given that they’ll raise electricity demand and increase power-plant emissions, will they really reduce overall ­greenhouse-gas production? It turns out that plug-ins always result in lower emissions than conventional cars do, and they beat regular hybrids handily–except when the electricity comes from coal (the source of 43 percent of U.S. electricity), according to a study. But as gasoline comes from dirtier oil sources, such as tar sands, plug-ins may win even when powered indirectly by coal, one study author says.